Researchers have noticed an intriguing transformation in the dynamics of this mutually beneficial interaction. In 2015, on Yakushima Island, Japan, scientists documented a low-ranking male monkey attempting to mate with two female deer. This was one of the first instances of such behavior being observed. While one deer appeared unfazed by the monkey’s sexual advances, the other shook the primate off and fled.
The most plausible explanation, according to the study’s authors, is mate deprivation. Male macaques with limited access to females are more likely to engage in such behavior. The researchers even captured video footage of this unexpected interaction.
Another group of researchers examined the peculiar conduct displayed by Japanese monkeys between 2012 and 2015 in Minoo, Japan, located north of Osaka. They recorded a total of 258 encounters between monkeys and deer. Interestingly, only juvenile female macaques were observed mounting sika deer, which closely resembled documented instances of sexual intercourse among teenage female macaques.
From these studies, the researchers concluded that there were no significant differences between female monkey interactions and the mounting behavior exhibited with the deer. Additionally, they discovered that only male deer could engage in effective sexual relations. Adult male deer, in particular, appeared unconcerned with monkeys on their backs.
Lead author Noëlle Gunst proposes several theories to explain the underlying causes of these sexual monkey-deer interactions. One theory suggests that male macaques can be aggressive during intercourse, prompting females to seek alternative ways to fulfill their needs—such as riding on the back of a sika deer. Another hypothesis suggests that the monkeys experience some form of stimulation while riding and grooming the deer.
Gunst elaborates, stating that “juvenile female macaques may first experience genital stimulation during these heterospecific playful interactions with deer playmates, then seek similar sexual reward with deer mates during the surge of sex steroid hormones characteristic of the adolescence period, particularly when sexually deprived conspecific male mates.”
It is plausible that the teenage female monkeys are exploring their sexuality, as males rarely accept them as sexual partners. However, according to the research, the most likely explanation is that female macaques are practicing for future sexual encounters with male monkeys.
The authors note that none of these theories are mutually exclusive and suggest that this behavior may represent a novel tradition among Japanese monkeys. Gunst concludes, “Each of them might be responsible for a portion of the event. We proposed that the monkey-deer sexual encounters described in our research might represent the early stages of a new Minoo behavioral tradition.”
